Electrostatic printer having selfregulating corona discharge



Feb. 27, 1968 M. MICHALCHIK 3,370,529

ELECTROSTATIC PRINTER HAVING SELF-REGULATING CORONA DISCHARGE Filed Aug-5, 1966 H5. C 8 [i L, 10

United States Patent 3,370,529 ELECTRQSTA'LIC PRKNTER HAVING SELF-REGULATING QQRONA DISCHARGE Michael Michaichilr, Bethpage, N.Y.,assignor to Fairchild Camera and instrument Corporation, a corporationof Delaware Continuation-in-pmt of application Ser. No. 427,294, Elan.22, 1965. This application Aug. 5, 1966, Ser. No. 570,638

17 Claims. (Cl. 1tl11) ABSTRACT OF THE DISCLQSURE Apparatus for forminga toner image on a plate having both conductive and insulative areas.The plate is subjected to a high current capability corona generatorhaving a self-regulating feature which maintains the ionized spacecharge between the corona generator and the plate below the levelnecessary for spurious electrical discharge therebetween. In addition,there is co-action between the corona generating system and the tonerwhich is introduced at high velocity into the ionized space. In effect,the toner becomes part of the high corona current flow and is depositedsolely on the conductive areas of the plate.

This is a continuation-in-part of application Serial No. 427,294, filedJanuary 22, 1965 now abandoned.

This invention relates in general to electrostatic image reproductiondevices and more particularly to methods and apparatus for producinghigh quality high speed printing on a variety of surfaces.

The prior art methods of electrostatic printing have numerousdisadvantages. Electrostatic printing press speeds in excess of 200 feetper minute have provided degraded reproductions of the image resultingfrom inadequate deposition of toner powder in the image areas and randomdeposition of toner powder in the background areas. The principalreasons for poor reproductions are lack of regulating means for securinguniform and adequate corona discharge during the deposition phase of thereproduction cycle; non-uniformity of charge imparted to the tonerpowder, in part due to the toner being introduced through the coronaelectrodes; slow transfer of toner powder from image plate to printingsurface due to the relatively low conductivity of image areas on theimage plate; introduction of toner, in the deposition phase of the cycleat slow speeds and; physical contact between the image plate and theimage support web during the image transfer phase of the cycle. Inaddition, high speed transfer of electrostatic images onto a variety ofuneven printing surfaces has hitherto not been practical.

Accordingly, it is the object of the present invention to provide aself-regulating corona control capable of supplying sufficient currentto insure adequate deposition of toner in the desired image areas.

A further object of the present invention is to provide an improvedapparatus for printing by electrostatic means that will provide increaseprinting speed and better quality than that attainable with existingelectrostatic printing apparatus.

Another object of the present invention is to provide an improved methodof electrostatic printing which permits printing by means of tonerpowders on a variety of printing surfaces, including uneven surfaces.

Another object of the present invention is to provide an improved methodof high speed electrostatic printing which avoids direct contact betweenprinting plate and receiving surface.

These and further objects and features of the present invention willappear from a reading of the following detailed description of oneembodiment of the invention, to be read in conjunction with theaccompanying drawings wherein like components in the several views areidentified by the same reference numeral.

In the drawings:

FIGURE 1 illustrates a conductive image plate with an insulatingphotoresist suitably prepared for reproduction.

FIGURE 2 illustrates schematically one embodiment of the depositionapparatus of the present invention in which toner powder is selectivelydeposited on a surface which includes insulating background andconductive image areas.

FIGURE 3 is a schematic illustration of one embodiment of the transferapparatus of the present invention in which the toner powder image istransferred to the printing surface.

FIGURE 4 illustrates the printing surface after the image has beentransferred thereto.

Briefly stated high speed electrostatic printing in accordance with thepresent invention involves the twophase process of deposition andtransfer. The deposition phase of the invention employs a conductiveplate coated with a photoresist, The plate is prepared in anyconventional manner in order to form thereon an image in the photoresistleaving a surface having both conductive and insulative areas. Theplate, which is connected to the power-supply-return through animpedance, is subjected to a controlled negative corona discharge whichforms an ionized space between the corona discharge source and the imagebearing plate. A toner powder is then introduced at high speed into thisionized space. In effect, the toner powder becomes part of the coronacurrent flow to the conductive image areas.

It is to be noted that in the following description the conductive areasof the plate are assumed to be positive with respect to the negativepotential of the corona producing electrode. Although, for ease ofunderstanding the invention is described in this manner, it will beunderstood that the invention is equally applicable to positive coronadischarge with a negative plate.

The transfer step of the present invention is accomplished bypositioning the plate such that the powder bearing surface of the platefaces a transfer electrode. A sheet of paper or any appropriate printingsurface is placed between the image plate and transfer electrode and apotential is applied between the image plate and the transfer electrode,the transfer electrode being of lower potential than image plate causingthe particles of powder to be transferred from the plate to the printingsurface. The powder particles may then be aflixed to the paper in anyconventional manner.

Referring now to FIGURE, 1, there is illustrated an image plate withinsulating photoresist bonded thereto in which image areas have beendefined by selectively removing portions of insulating photoresistleaving exposed conductive image areas in which toner powder may bedeposited. In the practice of the present invention it is necessary thatan image plate be first prepared containing the image which it isdesired to reproduce. In FIGURE 1 the image plate is identified ingeneral by reference numeral 1t and includes a conductive support 11having on one surface thereof an image to be reproduced which image isdefined by an insulating photoresist 13. Image area 14 may be formed byselectively removing insulating photoresist l3 bonded to support 11using any conventional process. For example, support 11 can be coatedwith insulating photoresist 13 to a thickness of approximately 0.4 milthen the photoresist 13 is photograpically exposed to the image it isdesired to reproduce. The exposed portions of photoresist 13 are thenetched away leaving conductive image areas 14 which correspond to thedesired image. Support 11 should have a maximum volume resistivity ofohm-centimeters to enable the corona current to increase to a range of50 to 300 microamperes per square inch of image area as required by thisinvention. This will be described in greater detail below.

Referring to FIGURE 2, a suitably prepared image plate 10 is subjectedto a corona discharge which as aforesaid, for case of understanding willbe described as negative. The negative charging by corona discharge isaccomlished by placing support 11 with defined image areas 14 thereonunder a high voltage electrode 17. The negative corona discharge formsan ionized space between corona electrode 17 and support 11 which isconnected to ground as for example through a mounting means shown asdrum 24 on which support 11 is mounted. Ionization is actually a flow ofelectrons between support 11 and corona electrodes 17, which flow issubstantially magnified by an increase in humidity or blowing tonerpowder 19 into the ionized space 15. In addition, the changes inconductivity on the surface of image plate 10, from insulatingphotoresist 13 to conductive support 11, in effect, reflected to coronaelectrodes 17, causes a non-uniform and at times inadequate ionizationin ion ized space 15. This non-uniformit has two main effects: first,toner powder 19 passed through ionized space 15, will not have a uniformcharge and will have a greater tendency to attract to the insulatingphotoresist 13 on support 11 and; second, the non-uniform corona currentcauses uneven deposition in conductive image areas 14. These effects areespecially pronounced in a high speed electrostatic printing process. Itthe magnitude of the voltage from the power supply applied to coronaelectrodes 17 is increased, in order to increase the corona current, itis likely that arcing will occur between the corona electrodes 17 andsupport 11 when the toner powder 19 is introduced between them. Sucharcing would result in poor reproductions and possible damage to support11.

This invention provides means that not only allows increased coronacurrent without increasing the corona supply voltage but also providesself regulation of this current to compensate for any transient effectssuch as increased humidity. Positioning a conductive shield 16 near thecorona electrodes 17, causes an increase in corona current flow withoutincreasing the voltage sup plied to corona electrodes 17. This isdesirable as a current between and 300 miero-amperes per square inch ofconductive image area 14 is required by this invention, the optimumvalue of the current being a function of and increasing with an increasein printing speed. If the current falls too low it degrades the printingquality, if too high it may result in arcing between support 11 andelectrodes 17.

As noted above, the conductive shield is employed as part of aself-regulating current control system. Conductive shield 16 acquires anelectrical charge induced by the corona discharge during operation. Thischarge accumulates until a chosen level is reached causing a spark tobridge the gap between spaced electrodes 21 connected between shield 16and ground 18 which discharges conductive shield 16. The desiredbreakdown voltage can be chosen by selecting the spacing betweenelectrodes 21.

When toner powder 19 is blown between corona electrodes 17 and imageplate 10 in the presence of ionization, the quiescent ionization currentis substantially increased causing the current to shield 16 to beincreased to some lesser extent. However, before the ionization currentbetween corona electrodes 17 and support 11 reaches the arcing level,shield 16 will be discharged through electrodes 21 thereby reducing theoverall ionization level below that at which arcing would occur betweencorona electrodes 17 and support 11. Reduction of the ionization levelin this manner also prevents an excessive charge from building up oninsulating photoresist 13 bonded to support 11 thereby reducing thelateral surface current on insulating photoresist 13 which breaks downthe insulating properties of these surfaces and causes toner powder 19to adhere to nonimage areas.

Once the ionized space is created, toner powder 19 is blown into ionizedspace 15 beneath corona electrodes 17, tangentially to image plate 10mounted on drum 24, by blower means 25. In accordance with the inventionblower means imparts a minimum velocity of 300 feet per minute to tonerparticles 19. Velocities of up to 20,000 feet per minute have been usedsuccessfully. It is to be noted that the high velocity imparted to tonerpowder 19 is instrumental in imparting a uniform negative charge to thistoner powder 19 not attainable at lower velocities. The uniform chargeon toner powder 19 tends to reduce the deposition of toner powder 19 onthe insulated photoresist 13. Once powder particles 19 have becomenegatively charged, they are drawn to image areas 14 of support 11. Itis to be noted that support 11 is electrically positive with respect toelectrodes 17 because corona electrodes 17 are connected to the negativeside of power supply 20 and support 11 is closer to ground or referencepotential.

The toner powder 19 may be made of a non-insulating material with amaximum volume resistivity of 10 ohm- 5 centimeters or of an insulatingmaterial with a minimum volume resistivity of 10 ohm-centimeters.Without limiting the scope of this invention, satisfactory operatingparameters for a. preferred configuration has shield 16 spaced A-inchfrom corona electrodes 17 which, in turn, are 0 arrayed in a circularsegment 1 inch from the printing plate 10. This configuration isoperable with 9000 volts applied to corona electrodes 17 having adiameter 3.5 mils, electrodes 21 being spaced 1% inches apart.

Support 11 is connected to ground 18 through an imfunction of a sparksuppressor, reducing the voltage difference between corona electrodes 17and support 11 thereby reducing the likelihood of electrical dischargebetween corona electrodes 17 and support 11. If electrical dischargebetween corona electrodes 17 and support 11 does occur, resistor 27limits the discharge current preventing damage to support 11.

One explanation of why resistor 27 acts as a spark suppressor ratherthan as a simple resistor is due to the fact that the current andvoltage On image plate 10 are out of phase with each other. Anotherexplanation for this spark suppression function is that prior to actualarcing between corona electrodes 17 and support 11 there is an increasein corona current. This current passes through resistor 27 causingsupport 11 to acquire a more negative voltage with respect to groundthereby reducing the voltage difference between corona electrodes 17 andsupport plate 11 below that at which arcing occurs. The optimum value ofresistor 27 varies inversely with the size of sup- 5 port 11. A range of10 to 10 ohms was found to be the optimum range for use with a squareinch support 11. Values of zero to 10 ohms have been used with varyingdegrees of success for this function, depending upon the support.

A high speed printing process requires that toner powder 19 be quicklyremoved from the support 11. This operation is readily accomplished bythe apparatus of this invention due to the high conductivity of theimage areas 14 as opposed to the relatively high insulating propertiesof image area surface used in the prior art. Printing speeds of up to20,000 feet per minute can be obtained with this process.

Once toner powder 19 has been deposited in image areas 14 011 support11, the deposition phase of the present invention has been completed andthe second or transfer phase can be initiated. Referring to FIGURES 3and 4, in the transfer phase of the invention, the toner powder 19 nowin the form of a powder image is to be transferred to a printing surface28 (FIG. 4). The image plate 5 10 is positioned opposite the transferelectrode 29. If

pedance shown as resistor 27. Resistor 27 performs thedesired, thetransfer electrode 29 may be coated with an insulating film 31 toprevent arcing between transfer electrode 29 and support 11. A printingsurface 28 of any reasonable thickness or irregularity is placed betweentransfer electrode 29 and image plate 10. When printing surface 28 is inposition, a reverse polarity of approximately 1500 volts is appliedbetween image plate and transfer electrode 29, and across printingsurface 28. Voltages as high as 8000 volts have been successfullyemployed when transfer electrode 29 has been coated with an insulatingfilm 31. In applying the reverse potential transfer electrode 29 may bepositive or negative with respect to support 11 but in any event, it isof a different potential, i.e. more negative or more positive thansupport 11 so that an electric field is created between the twosurfaces. In this manner the toner powder 19 forming the toner powderimage 12 is transferred from support 11 to printing surface 28 formingtransferred areas 12 corresponding to the image to be reproduced.

As indicated above for transfer of the toner powder image 12, it isnecessary that an electric field be created between support 11 andtransfer electrode 29. Thus, either one may be positive and the othernegative. If toner powder 19 is deposited with a negative corona,transfer electrode 29 should be positive or at least more positive thansupport 11. If toner powder 19 is deposited with a positive corona,transfer electrode 29 should be negative or at least more negative thansupport 11. In any event a voltage is applied between transfer electrode29 and support 11 so as to create an electric field between them andcause transfer of toner-powder image 12.

Once powder image 12 has been transferred to printing surface 28printing surface 28 is removed from between support 11 and transferelectrode 29. The powder image 12 is then fixed to the printing paper28. This final step in the practice of the invention may be accomplishedin any well-known manner, as for example, by heat, resinous overlay etc.

While what has been shown and described is believed to be the best modeand a preferred embodiment of the invention, modifications andvariations can be made therein as will be clear to those skilled in theart, without departing from the spirit of the invention and consequentlythe scope of the invention is intended to be limited solely by theappended claims.

What is claimed is:

1. Electrostatic printing apparatus capable of producing reproductionsfrom an image plate having a conductive support and at least a portionof one surface of said support bearing non-conductive material on thenon-image portions thereof, comprising a first station having firstmounting means for mounting said plate, said mounting means electricallygrounding said plate, a conductive shield mounted on said apparatus andinsulated from ground, said shield spaced from said plate, control meansconnected to said shield for automatically connecting said shield toground when the potential of said shield exceeds a predetermined levelwith respect to ground, a corona electrode mounted on said apparatus andinsulated from ground, said corona electrode spaced between and apartfrom said image plate and said shield, means for connecting a highvoltage power supply to said corona electrode to create a coronadischarge field between said corona electrode and said plate, means forintroducing toner powder into the space between said corona electrodeand said plate at a high velocity, said corona discharge field causingat least a portion of said toner powder to be deposited onto the imageareas of said plate, a second station having second mounting means formounting said plate, said means electrically grounding said plate, meansfor moving said plate from said first station to said second station, atransfer electrode mounted on said apparatus at said second station andinsulated from ground, said transfer electrode being spaced from saidplate when said plate is at said second station, means for placing aprinting surface between said plate and said transfer electrode, andmeans for connecting a high voltage powersupply between said transferelectrode and said plate so that the potential applied to said transferelectrode is opposite in polarity to the potential applied to saidcorona electrode, whereby the toner powder deposited on the imageportions of said plate at said first station are transferred from saidplate to said printing surface at said second station.

2. The device as defined in claim 1, wherein said first and secondmounting means are electrically connected to ground through resistors.

3. The device as defined in claim 1, wherein said first and said secondmounting means are a single mounting means movable between said firstand second stations.

4. The device as defined in claim 3, wherein said single mounting meansis electrically connected to ground through a resistor.

5. The device as defined in claim 1, wherein said control means is anoff-on device which is switched from the non-conducting state to theconducting state when the potential induced on said shield reaches achosen level and automatically returns to the non-conducting state whenthe discharge current falls below the conductive state sustaining levelof said off-on device.

6. The device as defined in claim 1, wherein said control means iscomprised of a pair of spaced apart electrodes.

7. The device as defined in claim 1, wherein said means for introducingsaid toner powder is a blower means capable of imparting a velocity ofbetween 300 and 20,000 feet per minute to said toner powder.

8. The device as defined in claim 1, whereby said corona discharge fieldproduces a corona current between said corona electrode and saidconductive support of at least 50 micro-amperes per square inch of imagearea on said image plate when said image plate is at said first station.

9. The device as defined in claim 1, wherein said conductive support hasa maximum volume resistivity of 10 ohm-centimeters.

10. The device as defined in claim 1, wherein said toner powder is madeof non-insulating material with a maximum volume resistivity of 10ohm-centimeters.

11. The device as defined in claim 1, wherein said toner powder is madeof an insulating material with a minimum volume resistivity of 10ohm-centimeters.

12. Electrostatic printing apparatus capable of transferring an image toan image plate having a conductive support and at least a portion of onesurface of said support bearing non-conductive material on the non-imageportions thereof, comprising means for mounting said plate, saidmounting means electrically grounding said plate, a conductive shieldmounted on said apparatus and insulated from ground, said shield spacedfrom said plate, control means connected to said shield forautomatically connecting said shield to ground when the potential ofsaid shield exceeds a predetermined level with respect to ground, acorona electrode mounted on said apparatus and insulated from ground,said corona electrode spaced between and apart from said image plate andsaid shield, means for connecting a high voltage power supply to saidcorona electrode to create a corona discharge field between said coronaelectrode and said plate and means for introducing toner powder into thespace between said corona electrode and said plate at a high velocity,said corona discharge field causing at least a portion of said tonerpowder to be deposited onto the image areas of said plate.

13. The device as defined in claim 12, wherein said mounting means iselectrically connected to ground through a resistor.

14. The device as defined in claim 12, wherein said control means is anolfon device which is switched from the non-conducting state to theconducting state when '7 the potential induced on said shield reaches achosen level and automatically returns to the non-conducting state whenthe discharge current falls below the conductive state sustaining levelof said off-on device.

15. The device as defined in claim 12, wherein said control means iscomprised of a pair of spaced apart elec' trodes.

16. The device as defined in claim 12, wherein said means forintroducing said toner powder is a blower means capable of imparting avelocity of between 300 and 20,000 feet per minute to said toner powder.

17. The device as defined in claim 12, wherein the spacing between saidcorona electrode and said conductive support is related to the voltagelevel of the power supply so as to provide a corona current of at least50 micro-amperes per square inch of conductive image area on said imageplate.

References Cited STATES PATENTS Pethick 10 Landrigan et al. 101Steinhilper 101 Haas 101 Jarvis 101 Adams et al. 101 Jarvis 101 Nail 101Childress et a1. 101 Rarey et al. 101 Javorik et al. 101 Rarey 101PULFREY, Primary Examiner.

E. S. BURR, Examiner.

